You can bring it up, but it may not be a complete defense to your breach of the lease.
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Whether the landlord can retake the property and still hold you liable for continuing rent depends upon your lease. Case law provides that if the the the landlord terminates a residential lease, then he has no right to continue to claim unpaid rents unless the lease allows him to do so. Even there, the landlord still has to mitigate his damages. If you r ex-roommate is still living there and paying the rent, then to claim future rents from you seems to be a double recovery. Good luck.
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